The present invention relates to a high-frequency multilayer circuit substrate used in a microwave band and employing a via hole for connection between circuit layers.
There has conventionally been a high-frequency multilayer circuit substrate, in which transmission lines are connected by way of a via hole provided for the improvement in performance of a high-frequency circuit and for high-density mounting. As reduction in size and increase in transmission signal frequency of this multilayer circuit substrate have been needed, a transmission loss due to signal wave reflection in the transmission lines and a via hole connecting portion has become a more serious trouble. In contrast to this, according to Japanese Patent Laid-Open Publication No. HEI 11-15037, characteristic impedance matching of the via hole connecting portion is achieved by providing a plate-shaped ground conductor of a specified width with a specified interval provided apart from a transmission line via hole or providing a columnar ground conductor with a specified interval provided apart from the transmission line via hole.
The above-mentioned high-frequency multilayer circuit substrate is required to control the characteristic impedance of the via hole connecting portion by adjusting the interval between the ground conductor and the via hole when providing characteristic impedance matching of the via hole connecting portion. However, in terms of a substrate manufacturing process, the circuit substrate manufacturing requires finer precision as the interval between the via hole that penetrates the substrate and the ground conductor becomes smaller in the case of either the plated-shaped ground conductor or the columnar ground conductor, disadvantageously causing a manufacturing cost increase.